Initial draft said this should be fine:
https://drafts.csswg.org/css-sizing/#column-sizing

But multcol spec disagreed:
https://www.w3.org/TR/css-multicol-1/#propdef-column-width

Kind of weird.

Removing this would be simple in code, only this (and one test):

		$props['column-width'] = new Alternative( array_merge(
			[ $matcherFactory->length(), $auto ],
			// Additional values from https://www.w3.org/TR/2019/WD-css-sizing-3-20190522/
			$this->getSizingAdditions( $matcherFactory )
		) );

It is mentioned in the initial spec in the code:
https://www.w3.org/TR/2019/WD-css-sizing-3-20190522/#column-sizing

What is the procedure here? Should it be kept for backward compatibility, or can we just remove it? I assume template styles using column-width: max-content; will not save correctly after removing that line. Not sure if they will render.

In T374292#10166066, @Nux wrote:

What is the procedure here? Should it be kept for backward compatibility, or can we just remove it? I assume template styles using column-width: max-content; will not save correctly after removing that line. Not sure if they will render.

I would expect concerns around a security-sensitive software package to imply removal in the general case.

Anyway, there is no procedure. They will not save correctly post-removal. I do not know about rendering either. I suppose that forms at least part of my motivation with tasks such as T354228 and T162379.

A quick global search looking only for and not e.g. \s for max-content/min-content in column-width didn't find anything. I didn't look in columns. So it is possible that there is no-one using these keywords in that context.

If the specs do not match, probably a bug should be submitted to the standardization committee. It's possible during implementation of css-sizing-3 that they were unable to get things to work, though max/min-content seems simple enough given the implementation note in css-sizing-3. Or that it was dropped because no browsers do support the added keywords.

Change #1074721 had a related patch set uploaded (by Eccenux; author: Eccenux):

[css-sanitizer@master] Support for CSS Box Sizing Level 4

https://gerrit.wikimedia.org/r/1074721

OK, I searched on plwiki in our template using column layout -- also no usages of (min|max|fit) in width param.

Added to the Box Sizing path (there would probably be conflicts if this was in two patches). Also this is kind of related in that:

Level 3 says:

The following features are at-risk, and may be dropped during the CR period:

• Additions to column-width

Level 4 removed it completely (now part of CSS Multi-column Layout Module Level 1)

In T374292#10166188, @Izno wrote:

In T374292#10166066, @Nux wrote:

What is the procedure here? Should it be kept for backward compatibility, or can we just remove it? I assume template styles using column-width: max-content; will not save correctly after removing that line. Not sure if they will render.

A quick global search looking only for and not e.g. \s for max-content/min-content in column-width didn't find anything. I didn't look in columns. So it is possible that there is no-one using these keywords in that context.

Did a bit more detailed look, did not see anything in global search. Example query.

In T374292#10166232, @Nux wrote:

Added to the Box Sizing path (there would probably be conflicts if this was in two patches).

I think correcting this issue can be fixed before correcting the other (and should be fixed separately). In git you can set the other to depend on a patch to fix this issue. (I don't know how to do that, just am like 95% certain it can be done. :^)